Method for producing a metal strip and device for reducing the risk of such a strip breaking

ABSTRACT

A method for producing a metal strip which reduces the risk of the strip breaking over the course of a path, the strip being conveyed under stress in the strip direction and in which, in an upstream feed path of the metal strip, defects in the edges of the strip are detected and then measures are taken to reduce the risk of these defects causing the strip to break, in which method the strip is provided, in the region of the detected defect, with at least one notch in the strip direction, at a distance which is further away from the edge of the strip than the maximum dimension of the defect transversely with respect to the strip direction.

FIELD OF THE INVENTION

The invention relates to a method for producing a metal strip whichreduces the risk of the strip breaking over the course of a path, thestrip being conveyed under stress in the strip direction and in which,in an upstream feed path of the strip, defects in the edges of the stripare detected and then measures are taken to reduce the risk of thesedefects causing the strip to break.

The invention also relates to a device for carrying out the novelmethod.

A method of the known type mentioned is used, for example, for treatinga cold-rolled steel strip in a continuous annealing line, although theinvention is not limited to this application.

When producing thin steel strip, as a rule the steel is successivelysubjected to what are known as hot and cold rolling treatments, in orderto achieve the ultimate desired thickness of a few tenths of amillimetre. However, these treatments cause metallurgical changes in thesteel which often make it less suitable for further processing.

BACKGROUND OF THE INVENTION

A conventional method of improving the processing properties consists inpassing the strip through an annealing furnace under a certain tensilestress.

A known problem with this method consists in the fact that the precedingsection of the production of the steel strip may cause defects in theedges of the strips, which defects may lead to the strip breaking in thefurnace when it is under stress.

Continuous annealing furnaces are highly complicated installations whichrequire a very considerable time to thread the strip through. If a stripbreaks, this loss of time causes enormous production losses. It istherefore essential to limit the occasions when the strip breaks to anabsolute minimum.

A conventional way of achieving this consists in detecting defects inthe edge of the strip in the feed path of the metal strip which precedesthe annealing section. An opto-electronic system is often used to dothis, in which case a defect detected by a camera is translated into anelectronic signal.

It should be noted that defects may comprise hairline cracks runningfrom the edge of the strip, but also loose flaps at the edge orrelatively coarse inclusions. Sudden changes in the width of the stripmay sometimes also be regarded as defects. A defect causes a localincrease in stresses in the strip if the latter is under stress. Thisincrease in stresses may initiate a crack which results in the stripbreaking.

The most simple measures which follow the detection of a defect consistin the movement of the strip being stopped and in a piece being removedfrom the edge of the strip at the location of the defect. Although sucha measure prevents the strip from breaking, it nevertheless leads to aloss of production, while furthermore a piece of strip has beensubjected to a different heat treatment, which may also lead todifferences in quality of the annealed strip.

SUMMARY OF THE INVENTION

The object of the invention is therefore to avoid the occurrence of astrip breaking as a result of a defect without stopping the movement ofthe strip and without requiring very complicated measures. To this end,the invention consists in the fact that, in the known method mentioned,the strip is provided, in the region of a detected defect, with at leastone notch in the strip direction, at a distance which is further awayfrom the edge of the strip than the maximum dimension of the defecttransversely with respect to the strip direction.

Forming a notch in the strip direction is considerably easier and lesssusceptible to speed limitations than removing a defect. The aim of thenotch formed is to prevent any edge crack caused by a defect from beingable to continue any further than the notch, so that this crack does notaffect the rest of the strip and there is no longer any possibility ofthe strip breaking.

The effect of one notch can be improved by forming a second notch, whilein the case of two notches the notch depth of each notch can be reduced.One possibility worth considering is that of forming two notchesopposite one another on either side of the strip. The choice of one ormore notches is dependent, inter alia, on the type of material of themetal strip and on the technical facilities provided by the installationemployed.

The most simple way of forming a notch consists in holding a sharp pinagainst the strip. However, this operation may itself give rise toundesirable defects in the strip, so that according to the invention ithas been found that preference is given to the use of one or morecutting wheels. A cutting wheel of this nature, having a sharp edge,presses the notch further into the material than the extent to which itis itself drawn into the latter. As a result, the edge of the notchbecomes smoother and the formation of a burr is avoided.

Other ways of forming a notch are also conceivable, however, such as forexample using a laser beam.

Obviously, a deep notch has the advantage that an edge crack which hasformed is blocked more effectively than where a shallow notch is used.On the other hand, an excessively deep notch risks causing alongitudinal crack at the location of this notch, with possible adverseeffects. According to the invention, it has been found that preferenceis given to a method in which the residual thickness of the metal stripat the location of the notch is between 10 and 40% of the stripthickness.

Since an edge crack initiated from a defect does not necessarily have topropagate precisely transversely to the strip direction, the notch mustbe sufficiently long. However, on the other hand, it is advantageousthat the strip should not be damaged unnecessarily. It has been foundthat the best results are obtained if each notch extends, in the stripdirection, upstream and downstream of the defect over a total lengthwhich is equal to the length of the defect measured in the stripdirection +1 to 3 times the distance between the notch and the edge ofthe strip. Furthermore, the simplest solution is obtained by making thenotch V-shaped with a vertex angle of less than 90°.

If, where a notch is present, a defect leads to an edge crack, it isconceivable that the latter will propagate through the notch. In thatcase, the result is a sudden change in strip width at the end of thenotch, which could itself in turn give rise to a fresh edge crack. Inorder to prevent this, it has been found to be effective, according tothe invention, for the notch depth to increase gradually, from the endsof the notch until it reaches its full depth, over a distance of between2 and 20 times the strip thickness. In this way, an edge crack whichpropagates through the notch will come up against continuously thickerstrip material before reaching the end of the notch.

It has already been noted above that the method is not limited to use onsteel strips which are passed through an annealing furnace after theyhave been cold-rolled. It could also be used on metal strips ofdifferent compositions and also on metal strips which are passed throughother treatment installations, such as a coating bath. Nevertheless, ithas been found that the invention provides very good results inparticular when used in the treatment of cold-rolled steel strip in acontinuous annealing line.

As has been noted, the invention also relates to a device for reducingthe risk of a strip breaking. This device may then be arranged in aninstallation comprising a feed system for a metal strip and a treatmentsystem, in which the metal strip is held under stress, which devicecomprises, in the feed system, a detector for detecting defects in theedges of the moving metal strip. In this case, according to theinvention, the novel device is furthermore provided with a system forforming at least one notch in the metal strip in the strip direction,which system furthermore comprises control means which can be used tocontrol the location, depth and length of the notch on the basis ofsignals transmitted by the detector.

Furthermore, the novel device may be designed in such a way that it issuitable for carrying out the diverse variants of the method asdescribed above.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained with reference to a number offigures which diagrammatically illustrate the use of the invention on acontinuous annealing line for steel strip. In the drawing:

FIG. 1 shows a diagrammatic illustration of an annealing line;

FIG. 2a and FIG. 2b show a cross section and a top view of a piece of asteel strip;

FIG. 3a shows a detail of how a notch is formed, in cross section;

FIG. 3b shows a front view of this detail.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, reference numeral 1 denotes a cold-rolled steel strip whichis unwound from a coiler 2 and, after it has been treated, is, woundonto a coiler 3. Reference numeral 4 diagrammatically depicts acontinuous annealing furnace. Before entering the annealing furnace,strip 1 passes through an entry buffer 5, and after leaving the furnaceit is passed through an exit buffer 6. Arrows diagrammatically indicatethat the volume of strip in the entry buffer 5 and in the exit buffer 6can be varied. This makes it possible to accommodate discontinuities inthe course of the strip without the passage of the strip into theannealing furnace 4 becoming irregular. Such irregularities in therunning of the strip may be caused if a new strip is being placed oncoiler 2 and has to be welded to the old strip in a welding installation7. In a similar way, a full coiler can be cut free from strip 1, afterwhich the strip has to be guided onto a new coiler.

A detector 8 is positioned upstream of entry buffer 5. The detector 8illustrated is positioned above one of the edges of the strip and is ofa type which views a halogen illumination from below the strip by meansof a camera which is positioned above the strip. The light received bythe camera is detected and translated into a crack or some other defect.A device 13 for forming a notch in the strip is situated downstream ofthe detector in the strip direction. This device 13 is controlled by thesignal transmitted by detector 8. Since this requires simple controlengineering, the details of this control are not described further inthe following text.

Since edge effects may occur in both edges of the strip, a detector 8and a device 13 will generally be arranged on either side of the stripin the vicinity of both edges.

FIG. 2a shows a cross-sectional view of a steel strip 1. FIG. 2b shows atop view of the same strip 1.

In FIG. 2b, a defect 9 in the edge is shown, which defect is depicted asa sharply tapering notch. When the strip 1 is placed under stress (cf.the arrows in FIG. 2b), there is a real risk of this notch resulting ina continuous crack which may lead to the strip breaking. FIGS. 2a and 2b furthermore show a V-shaped notch 10. This notch is formed in thelongitudinal direction of the strip and in the vicinity of the deepestpoint of defect 9. It has been found that a notch of this natureprevents a crack from being able to propagate transversely to the stripdirection. In the case illustrated, the depth and length of the notchare indicated diagrammatically. In a situation which occurs in practice,these dimensions will satisfy the condition explained in the precedingtext.

FIG. 3 shows a structure for forming the notch in strip 1, specificallyin cross section in FIG. 3a and in front view in FIG. 3b.

Reference numeral 11 denotes a plastic-coated roll over which strip 1 ispassed. FIG. 3b shows an edge defect 9 behind which a notch is to beformed in the strip. To do this, a cutting wheel 12 with a sharpV-shaped edge is positioned above the roll 11. Cutting wheel 12 can bemoved up and down in the direction of the arrow, so that it can be movedoff strip 1 but can also be pressed into the latter. Cutting wheel 12 ismounted so that it can rotate freely about an axle (not shown).

The location of the cutting wheel 12 in the axial direction of roll 11can be varied as a function of the depth of the edge defect 9 detected.Naturally, the notch will be formed as close as possible to the deepestpoint of such a defect. Both the location of the cutting wheel and itsdepth are easy to control from detector 8 as a function of the presenceand shape of the defect observed and detected. Since these are simplemovements which are rapidly able to follow a signal received, it is evenpossible to form a notch without having to reduce the strip velocity.

Obviously, numerous variants on this design are conceivable, inter aliadepending on whether a plurality of notches are desired in the strip.All these variants are obvious to the person skilled in the art and donot need to be explained further in order to be clearly understood.

What is claimed is:
 1. A method for producing a metal strip which reduces the risk of the strip breaking over the course of a path, the strip being conveyed under stress in the strip direction and in which, in an upstream feed path of the metal strip, defects in the edges of the strip are detected and then measures are taken to reduce the risk of these defects causing the strip to break, wherein the strip is provided, in the region of a detected defect, with at least one notch in the strip direction, at a distance which is further away from the edge of the strip than the maximum dimension of the defect transversely with respect to the strip direction.
 2. The method according to claim 1, wherein two notches are formed opposite one another on either side of the strip.
 3. The method according to claim 1 or 2, wherein the notch or notches is (are) formed using a cutting wheel (cutting wheels).
 4. The method according to claim 1, wherein the residual thickness of the metal strip at the location of the notch is between 10 and 40% of the strip thickness.
 5. The method according to claim 1, wherein each notch extends upstream and downstream of the defect, in the strip direction, over a total length which is equal to the length of the defect measured in the strip direction +1 to 3 times the distance between the notch and the edge of the strip.
 6. The method according to claim 1, wherein the notch is V-shaped, with a vertex angle of less than 90°.
 7. The method according to claim 1, wherein the notch depth increases gradually, from the ends of the notch until it reaches its full depth, over a distance of between 2 and 20 times the strip thickness.
 8. The method according to claim 1, wherein the method is used to treat cold-rolled steel strip in a continuous annealing line.
 9. A device for reducing the risk of a strip breaking in an installation comprising a feed system for a metal strip (1) and a treatment system (4), in which the strip (1) is held under stress, which device comprises, in the feed system, a detector (8) for detecting defects in the edges of the moving strip, wherein a system (12) is also provided for forming at least one notch in the strip in the strip direction, which system furthermore comprises control means which can be used to control the location, depth and length of the notch on the basis of signals transmitted by the detector (8).
 10. A device for producing a metal strip which reduces the risk of the strip breaking over the course of a path, the strip being conveyed under stress in the strip direction and in which, in an upstream feed path of the metal strip, defects in the edges of the strip are detected and then measures are taken to reduce the risk of these defects causing the strip to break, wherein the strip is provided, in the region of a detected defect, with at least one notch in the strip direction, at a distance which is further away from the edge of the strip than the maximum dimension of the defect transversely with respect to the strip direction, comprising: a feed system for the metal strip (1), wherein the feed system comprises, a detector (8) for detecting defects in the edges of the moving strip, a treatment system (4) in which the strip (1) is held under stress, and a cutter system (12) for forming at least one notch in the strip in the strip direction is located downstream of the detector in the strip direction, the system comprising control means capable of controlling the location, depth and length of the notch on the basis of signals transmitted by the detector.
 11. The device of claim 10, wherein the cutter system comprises a cutting wheel.
 12. The device of claim 10, wherein the cutter system comprises a plastic coated roll (11) and a cutting wheel located adjacent the plastic coated roll, wherein the plastic coated roll (11) and the cutting wheel are located to pass the strip therebetween. 